Take posterior cornea into account when correcting astigmatism

Surgeons should know the various ways to correct astigmatism to increase patient satisfaction.

Issue: April 10, 2015

Understanding the role of the posterior corneal surface and getting the right measurements in astigmatism correction during cataract surgery can affect overall patient satisfaction, according to a presenter at Hawaiian Eye 2015.

John A. Hovanesian, MD, FACS, OSN Cataract Surgery Section Editor, recommended learning varying techniques in order to become a better cataract surgeon.

John A. Hovanesian

“Really knowing how to [correct astigmatism] — and there are a lot of ways now, with toric IOLs, femtosecond laser and an old-fashioned [astigmatic keratotomy] — it’s something everybody should know how to do,” he said.

Patient survey

A recent in-house survey on patient satisfaction, consisting of 200 patients from Hovanesian’s office, showed that 80% of patients were “extremely satisfied” when residual astigmatism was 0.5 D or less. Patient satisfaction decreased to 56% if there was more than 0.5 D of residual astigmatism.

With the use of astigmatic keratotomy and limbal relaxing incisions (LRIs), Hovanesian said in his office 51% of patients achieve 0.25 D or less of residual cylinder while 70% achieve less than 0.5 D.

“I encourage you to learn this technique if you want to become a better cataract surgeon because even with incisional LRIs, these are the results,” he said. “So it really largely depends upon getting the right measurements.”

Central cornea

In order to achieve success in astigmatism correction, the first step is to understand the patient’s surgically induced astigmatism and corneal astigmatism, Hovanesian said. He said astigmatism is best measured in the central 2 mm to 3 mm of the cornea.

Blade depth also affects the amount of astigmatism being corrected, and it is important for the blade to be perpendicular to the corneal surface.

Posterior corneal astigmatism

According to Douglas Koch’s landmark study, approximately 90% of eyes have hidden posterior corneal astigmatism that functionally acts like against-the-rule (ATR) cylinder, adding steepness at 180° of about 0.5 D. Therefore, the additional 0.5 D can be added to the pre-existing topographic astigmatism during planning, Hovanesian said.

Additionally, most patients have a shift in their astigmatism over time toward ATR astigmatism, which separately amounts to about 0.4 D, typically happening over about 10 years, whether patients had surgery or not.

“If we’re treating with-the-rule (WTR), we are going to tend to overcorrect by about 0.5 D,” he said. “Whereas if we’re treating ATR, we’ll tend to undercorrect by about 0.5 D because there’s about a half diopter there that we’re not measuring.”

Hovanesian recommended correcting about 0.5 D to 0.7 D less when treating WTR astigmatism than topography alone would indicate. When treating ATR astigmatism, increase the correction by approximately 0.5 D, adding more than the measured amount of astigmatism would indicate, he said.

“You can do LRIs. You can do [astigmatism correction] with a laser … you can do them with an incision and of course you can use toric IOLs to correct astigmatism. Use the tools you have available in order to give patients the most satisfactory result, but also factor in the result of the posterior cornea,” Hovanesian said. – by Kristie L. Kahl

For more information:

John A. Hovanesian, MD, FACS, can be reached at Harvard Eye Associates, 24401 Calle De La Louisa, Suite 300, Laguna Hills, CA 92653; email: drhovanesian@harvardeye.com.
Disclosure: Hovanesian reports no relevant financial disclosures.